In general, hammer bits are used to perform drilling of the ground for the study of the structure and growth of the earth strata. A variety of different hammer bits having different specifications and structures are selected and used depending on a stratum condition or excavation depth.
Excavation methods using the hammer bits are classified, in accordance with whether a reclamation pipe is applied, into a direct excavation method and an indirect excavation method.
In the direct excavation method, the stratum is excavated by the hammer bit mounted on a hammer drill without using the reclamation pipe. The direct excavation method is generally used when the stratum is relatively stable or an excavation hole is not deep enough such that an excavated hole is not collapsed.
In the indirect excavation method, the stratum is excavated in a state where the hammer bit and the hammer drill are inserted into the reclamation pipe. At this point, as the hammer bit excavates the stratum, the reclamation pipe is inserted into an excavated hole together with the hammer bit. The indirect excavation method is generally applied when the stratum is relatively unstable or the excavation hole is deep.
In the indirect excavation method, the hammer bit bores a hole at a portion under the reclamation pipe such that the hole has a lager diameter than the reclamation pipe so that the reclamation pipe can be inserted into the excavated hole. As the excavation depth is increased, a load applied to the hammer bit is increased due to the increase of the pressure applied by a load of the reclamation pipe.
When the hammer bit rotates for the excavation, wing bits are unfolded by being caught by a rock or soil around thereof. At this point, a bit body is provided at an edge thereof with a plurality of folding spaces in which the respective wing bits are folded. The wing bits are coupled in the respective folding spaces by respective hinge shafts to rotate at a predetermined angle.
In addition, when the excavation is finished, the hammer bit rotates in an opposite direction to fold the wing bits such that the hammer bit is down-sized to be smaller than an inner diameter of the reclamation pipe. At this point, since an overall outer diameter of the hammer bit becomes less than the inner diameter of the reclamation pipe, the hammer bit can be withdrawn through the reclamation pipe.
However, sludge such as excavated soil or crushed rocks may be filled in the folding spaces of the bit main body during the excavation. In this case, since the wing bits are not folded even when the hammer bit rotates in the opposite direction after the excavation is finished, the hammer bit cannot be withdrawn.
Further, since the wing bits are coupled to the bit body by the hinge shafts, the loads applied to the wing bits are concentrated on the respective hinge shafts. In addition, as the excavation depth of the hammer bit is increased, the load applied to the hinge shaft of each of the wing bits by the reclamation pipe is increased. Therefore, the chance of damaging the wing bits is increased.
As the chance of damaging the wing bits is increased, the excavation depth of the hammer bit may be limited. In addition, when the hammer bit is damaged during the excavation, the withdrawal of the hammer bit may be abandoned or another location may be excavated.
Further, since the wing bits must be coupled to the bit body by the hinge shafts, the assembling time and cost for the hammer bit may be increased.